Taxol is a naturally occurring diterpenoid which has great potential as an anti-cancer drug, and which has shown activity in several tumor systems. Taxol was first isolated and its structure reported by Wani, et al., in "Plant Anti-Tumor Agents. VI. The Isolation and Structure Of Taxol, A Novel Anti-Leukemic And Anti-Tumor Agent From Taxus brevifolia, "J. Am. Chem. Soc., 1971, 93, 2325. Taxol is found in the stem bark of the Western Yew, Taxus brevifolia, as well as in T. baccata and T. cuspidata.
The biological activity of taxol is related to its effect on cell division. Taxol promotes formation of microtubules that form the mitotic spindle during cell division. However, taxol prevents depolymerization of the tubulin forming the microtubules of the mitotic spindle, which is essential for cell division to take place. Thus, taxol causes cell division to stop. Taxol's mechanism is unique since it promotes the formation of tubulin polymers, whereas other anti-cancer drugs, such as vinblastine and colchicine, prevent microtubule formation.
Extensive testing of taxol has not been performed because taxol is in short supply and has not yet been successfully synthesized. Preliminary studies have shown that taxol may have marginal activity in acute leukemia and melanoma, and some activity has been noted in other tumors. Further, studies by McGuire et al. found taxol to be an active agent against drug-refractory ovarian cancer. See "Taxol: A Unique Antineoplastic Agent With Significant Activity In Advanced Ovarian Epithelial Neoplasms," Ann. Int. Med., 1989, 111, 273-279, herein incorporated by reference. However, due to the low water solubility of taxol, doses had to be delivered as infusions diluted in aqueous dextrose solutions.
It should be noted that in phase 1 clinical trials, taxol itself did not show excessive toxic effects, but severe allergic reactions were caused by the emulsifiers administered in conjunction with taxol to compensate for taxol's low water solubility. In fact, at least one patient's death was caused by an allergic reaction induced by the emulsifiers. Therefore, researchers have attempted to create water soluble derivatives of taxol which retain their anti-neoplastic and anti-cancer activity.
With reference to FIG. 1, the structure of taxol is illustrated along with a .sup.1 H nuclear magnetic resonance (NMR) spectrum of a taxol sample. The NMR signals are well separated and cover the region from 1.0 to 8.2 ppm. For simplicity, the spectrum is divided into three regions: a first region between 1.0 and 2.5 ppm formed by strong 3-proton signals of the methyl and acetate groups as well as complex multiplets caused by certain methylene groups; a second region between 2.5 and 7.0 ppm represents the signals observed from most of the protons on the taxane skeleton and the side chain; a third region between 7.0 and 8.2 ppm is formed by the signals from the aromatic protons of the C-2 benzoate, C-3' phenyl and C-3' benzamide groups. The peaks of the NMR spectrum in FIG. 1 are labeled according to the number of the carbon in the taxol structure to which the protons including the signals are attached.
Magri and Kingston reported on the biological activity of taxols substituted at the C-2' and C-7 positions in order to make them more water soluble. See "Modified Taxols, 4..sup.1 Synthesis And Biological Activity Of Taxols Modified In The Side Chain," Journal of Natural Products vol. 51, no. 2 pp. 298-306, March-April 1988, herein incorporated by reference. A 2'-(t-butyldimethylsilyl)taxol was synthesized and found to be essentially inactive; this was taken as an indication of the need for a free hydroxyl group at the 2' position of the taxol side chain for biological activity. Further, acyl substituents at the 2' position in 2'-acetyltaxol and 2',7-diacetyltaxol were readily hydrolyzed under in vivo conditions, and both showed activity in a cell culture bioassay. The lability of the acyl substituents at the 2' position suggested that 2'-acetyltaxols could serve as pro-drug forms of taxol. (Generally, a prodrug is a compound which exhibits pharmacologic activity after biotransformation.)
Magri and Kingston reported that two taxols with increased water solubility were prepared, 2'-(.beta.-alanyl)taxol: ##STR1## and 2'-succinyltaxol: ##STR2## The 2'-(.beta.-alanyl)taxol was found to be active in vivo and in vitro, but was unstable. The 2'-succinyltaxol, prepared by the treatment of taxol with succinic anhydride, had a much diminished P-388 in vivo activity as compared with taxol. Thus, research efforts were concentrated on other derivatives of taxol which did not suffer from instability, or inactivity in vivo or in vitro.
Deutsch et al., in "Synthesis Of Congeners And Prodrugs. 3..sup.1 Water-Soluble Prodrugs Of Taxol With Potent Antitumor Activity," J. Med. Chem. 1989, 32 788-792, herein incorporated by reference, reported that salts of 2'-succinyltaxol and 2'-glutaryltaxol had improved antitumor activities when compared to the free acids. Since these researchers believed that salts prepared with different counterions often have substantially different properties, a variety of 2' substituted taxol salts were synthesized and tested. Triethanolamine and N-methylglucamine salts of the 2' substituted taxol derivatives showed greatly improved aqueous solubility and had more activity than sodium salts. Further, a series of 2'-glutaryltaxol salts were found to have higher activity than their 2'-succinyltaxol analogs. In particular, the taxol salt resulting from the coupling of 2'-glutaryltaxol with 3-(dimethylamino)-1-propylamine using N,N'-carbonyldiimidazole (CDI), demonstrated good solubility and bioactivity.
In addition to increasing the solubility and bioactivity of taxol, it is desirable that the taxol derivatives formed have increased stability to prolong their shelf life. It is believed that salts of taxol esters are very susceptible to base hydrolysis, and water-solubilizing groups, such as carboxylate salts or amine salts, tend to be basic. Thus, it is desired that neutral, water-soluble taxol derivatives be synthesized which also have improved or the equivalent activity to taxol. Organic sulfonate salts tend to be neutral or only slightly basic, and therefore, sulfonate salts of taxol esters should have improved stability. Further, due to the difficulties involved in synthesizing carboxylic and amine salts of taxol esters, it is desirable to find less expensive water-soluble taxol derivatives and processes for forming them.